1. Field of the Invention
The present invention generally relates to optical transmission modules and methods of forming such modules, and particularly relates to an optical transmission module which is used in such areas as optical communication, an optical LAN (local area network), and an optical interface, and to a method of forming such a module.
2. Description of the Prior Art
Devices such as light emitting/receiving devices used in optical transmission modules of the prior art differ depending on their applications. For example, in light of a future prospect that speed and capacity of information transmission will need to be further increased, there is mounting need for methods of parallel and high-speed transmission of light signals. Among such transmissions, long-distance transmission may require a combination of a semiconductor laser array and single-mode optical fibers. On the other hand, intermediate-to-short-distance transmission may require a combination of a light-emitting diode array and multi-mode optical fibers.
A first reference of the prior art, Japanese Laid-Open Patent Application No.4-361210, discloses a device connecting a light-emitting device array or a light receiving-device array with an optical fiber array and a method of implementing the same. In this first reference, a connecting device includes a first fixture which has a slanted slide surface and holds an optical fiber array, and a second fixture which has a slanted fitting surface which can support the slanted slide surface of the first fixture sliding thereupon and holds a light-emitting device array (or light-receiving device array). By placing and sliding the slanted slide surface on the slanted fitting surface, the light-emitting/receiving device array can be precisely positioned with regard to the optical fiber array.
A second reference of the prior art, Japanese Laid-Open Patent Application No.4-157405, discloses a light device module. In this second reference, light-emitting/receiving devices are connected via solder bumps to a wiring pad of a metal wiring pattern on a board, and optical fibers are fixed into v-shaped grooves formed on the board. In positioning the light-emitting/receiving devices relative to the optical fibers, positioning precision in the longitudinal and lateral directions with regard to the board is ensured by utilizing the self-alignment ability of the melted solder and by controlling the amount of the pasted solder. Also, a highly efficient light coupling is obtained by aligning the field patterns and inserting spherical lenses between the light-emitting/receiving devices and optical fibers. The optical fibers are positioned at a predetermined distance away from the spherical lenses so as to avoid damages caused by a physical contact.
Semiconductor lasers used for light transmission at practical levels as described above have a structure of an edge-emitting type. In order to efficiently lead a light signal transmitted from those lasers into a single-mode optical fiber, which has a core diameter in the order of microns, a highly precise positioning/connecting method must be devised. Also, in addition to the realization of the highly precise positioning and connecting method, light-signal transmission modules are required to be miniaturized and made light-weight. This is because existing facilities for copper wiring cables should also be used for optical fibers for the benefit of cost saving of the investment. Especially, a parallel data transmission in 1-byte units, which is used for a data bus and the like, imminently requires a larger capacity and miniaturization of such modules. The first and second references cited above are devised in order to response to such needs.
In the first reference, however, a guide board becomes necessary for aligning optical fibers. The guide board defines a predetermined array pitch for optical fibers and semiconductors such as a light-emitting device array or a light-receiving device array implemented on a heat sink for releasing heat. Thus, metal stems, mounts, or blocks for a fixing purpose become necessary, putting a limit on an extent to which the device is miniaturized. Also, the cost of the device is difficult to lower because of material costs of such components. Since this type of device has various components in a large number, inordinate efforts are required for the adjustment of each components with regard to x, y, and z axes and rotation axes. This leads to a difficulty in implementing such devices into each part of a module.
In the second reference, spherical lenses (or micro-lenses) are employed in order to efficiently lead light emitted from an light-emitting/receiving device into an optical fiber having a diameter in the order of microns. In this case, bump connections by using melted solder are employed to ensure a sufficient precision in positioning the devices. The solder bumps are created by depositing three metal layers of titanium/tungsten-alloy/copper on wiring pads formed on a light-emitting/receiving device chip, electroplating copper and solder (Pb-Sn, 96:6) on the bi-metal alloy, and reflowing at the end. Thus, there are cases in which connection failures are generated during a thermal cycle by a difference in thermal expansion coefficients between the solder bumps and a substrate, and in which remaining flux created during the reflow causes defects in the light-emitting/receiving device. Those problems add to the lack of reliability of the device. Also, since arrayed light-emitting/receiving devices increase the number of parameters of electrical control, ensuring the reliability becomes more difficult. The array structure adds to a difficulty in positioning the arrayed devices in the longitudinal and lateral directions by using the self-alignment ability of the melted solder and controlling the amount of the solder. Furthermore, using solder bumps puts a limit on cost reduction because of the cost of forming bumps.
Accordingly, there is a need in the field of optical transmission modules for an optical transmission module with a miniaturized size and a larger capacity which is reliable, operates efficiently, and can be produced at a low cost.
Also, there is a need for a method of forming such an optical transmission module.